1. Field of the Invention
The present invention relates to a method of preparing an acetal or a ketal and, more particularly, to a method of preparing an acetal or ketal by causing an aldehyde or ketone to react with an alcohol in the presence of a catalyst.
2. Description of the Prior Art
When used in a variety of reactions, an acetal group or a ketal group is less reactive than a carbonyl group and can be easily converted into a carbonyl group. For this reason, when an aldehyde or a ketone is to be used in a desired reaction, it is first converted into an acetal or ketal. After the reaction has taken place, the reaction product is then converted into an aldehyde or ketone. That is to say, the acetal or ketal is very important as a protection group of the carbonyl group. An acetal or ketal is also important as a source material for synthetic perfume.
An acetal or ketal can be obtained by any one of the following reactions: acetylene reacting with an alcohol, an organometallic compound with an orthoester, vinyl ether with an alcohol, or a gem-dihalide with a metal alcoholate.
A more popular method of preparing an acetal or ketal is one in which an aldehyde or a ketone is used as a starting material and converted into a corresponding acetal or ketal. This method employs a synthesis reaction, for example, in which an aldehyde or ketone is made to react with an orthoester. However, the orthoester is expensive and is not easily available.
Therefore, another synthesis reaction is more often employed, in which an aldehyde or ketone is allowed to react with an alcohol. This synthesis reaction is performed in a solvent such as benzene in the presence of an acid catalyst. Water, which occurs as a by product, is removed by azeotropic distillation. Typical acid catalyst is a mineral acid (e.g., sulfuric acid or a p-toluenesulfonic acid) or its derivative, although in recent years, an organic acid which is soluble in organic solvents has also come into use for the same purpose. Use of this method, however, presents the following problems:
First, since the synthetic reaction takes place in a homogeneous solution, a number of complicated operations must be performed, following the reaction, in order to isolate the product thereof. In particular, since the acid used as a catalyst is strong, a large quantity of alkaline water must be used to neutralize the solution, in order to separate the catalyst. Thus, a cooling operation is required, in order to eliminate the heat produced in the neutralizing process, and also a liquid separation operation.
Second, since a strong acid catalyst is used, the reaction vessel tends to become corroded, and the heating required in this reaction merely accelerates the corrosion process.
Third, since a strong acid is used as the catalyst, an aldehyde or ketone (e.g., 4-hydroxy-2-butanone) which is not stable in acidic conditions cannot be used as a starting material.